When producing documents of value, which within the framework of the present invention means bank notes, check forms, shares, identification documents, credit cards, flight tickets and other deeds and documents as well as labels, seals, packaging and other elements for protecting products, it is particularly important to make arrangements against tampering and/or to take measures that permit the detection of the authenticity. Apart from the features which can be detected or used when testing is made by machines, there also exist features, which can be applied to such documents by everybody without technical auxiliary means and without any particular specialist knowledge for an unambiguous detection of the authenticity.
One possibility to equip a security element, such as a security thread, with visually easily recognizable elements is described in EP 0 330 733 B1, in which a security thread is provided with gaps in an opaque layer, and which contains colouring and/or luminescent substances in the gaps. This security thread is embedded in security papers as a so called “window security thread”, i.e. it is woven in the paper during the sheet formation of the security paper, so that in regular distances it is freely accessible at the surface of the paper and fully embedded only in the intermediate areas.
So as to increase the optical strikingness of a security element and to emphasize the value of the object to be protected, the security elements often are equipped with silver or gold colour tones.
One possibility to obtain a gold-coloured coating is to vapor-deposit thin gold layers onto well reflecting grounds, such as e.g. aluminum or silver. But due to the high costs of the source material and the great technical efforts required to manufacture very regular layers, profitability is not given.
Alternatively, also gold bronzes have been applied by means of vapor deposition with evaporation boat or sputtering. When vapor-depositing with evaporation boat a wire is continuously fed into a hot boat. When the boat is hot enough the just fed piece of wire immediately evaporizes and completely transitions into the vapor phase. A substrate located thereabove is coated with just that composition of this piece of wire. Within the boats, however, little lakes are often formed out of the molten wire, which e.g. consists of a certain alloy. From these lakes the individual components of the alloy evaporate at different rates due to different vapor pressures, the proportion of ingredients therefore being changed in the deposited material and thus on the substrate. The initially set colour tone thus continuously changes across the coating length and width of the item to be vapor-deposited during the vapor deposition process.
Furthermore, also the sputtering technique is used. Here metal clusters are knocked off from a fixed target in the plasma, which condense on the substrate disposed thereabove. The composition of the coating can be kept relatively constantly, but sputtering is a very time intensive technique and thus of very low productivity.